Light emitting devices using a light emitting diode (LED) are mainly composed of a combination of an LED chip as an excitation light source and a fluorescent material. Further, a variety of luminescent colors can be achieved depending on the combination.
For white LED light emitting devices for emitting white light, for example, a combination of an LED chip for emitting blue light with a fluorescent material is used. As the fluorescent material, a yellow fluorescent material is mainly used which emits light complementary to blue, and with the LED chip, used as a pseudo white light LED. Besides, three-wavelength white LEDs have been developed which use an LED chip for emitting blue light, a green to yellow fluorescent material, and a red fluorescent material.
It is desirable for the white light emitting devices to achieve high color rendering properties, in particular, a high average color rendering index (Ra) in order to reproduce a color close to natural light. In addition, for lower power consumption, the devices require a balance with a high luminescent efficiency.